Polishing method and polishing apparatus for workpiece

ABSTRACT

A polishing method that can reduce influence of polishing liquid (e.g., slurry) on measuring of a film thickness of a workpiece and can achieve accurate measuring of the film thickness is disclosed. The polishing method includes pressing the workpiece against a polishing pad by a polishing head while polishing liquid is present on the polishing pad to polish the workpiece; during polishing of the workpiece, directing light from an optical sensor head onto the workpiece and receiving reflected light from the workpiece by the optical sensor head, the optical sensor head being disposed in the polishing table; determining a film thickness of the workpiece based on a spectrum of the reflected light; and during polishing of the workpiece, supplying cleaning liquid from a cleaning nozzle to a target position which is located just above the optical sensor head.

CROSS REFERENCE TO RELATED APPLICATION

This document claims priority to Japanese Patent Application No.2022-056134 filed Mar. 30, 2022, the entire contents of which are herebyincorporated by reference.

BACKGROUND

In a manufacturing process of a semiconductor device, various materialsare repeatedly formed in film shapes on a silicon wafer to form amultilayer structure. In order to form such multilayer structure, atechnique of planarizing a surface of an uppermost layer of themultilayer structure is becoming important. Chemical mechanicalpolishing (CMP) is used as such planarizing technique.

The chemical mechanical polishing (CMP) is performed by a polishingapparatus. This type of polishing apparatus generally includes apolishing table that supports a polishing pad, a polishing headconfigured to hold a workpiece (for example, a wafer having a film), anda polishing-liquid supply nozzle configured to supply a polishing liquid(for example, slurry) onto the polishing pad. When a workpiece ispolished, the surface of the workpiece is pressed against the polishingpad by the polishing head while the polishing liquid is supplied ontothe polishing pad from the polishing-liquid supply nozzle. The polishinghead and the polishing table are rotated to move the workpiece and thepolishing pad relative to each other, thereby polishing a film formingthe surface of the workpiece.

In order to measure a thickness of a non-metal film, such as adielectric film or a silicon layer, the polishing apparatus generallyincludes an optical film-thickness measuring device. This opticalfilm-thickness measuring device is configured to direct light, emittedby a light source, to the surface of the workpiece and analyze aspectrum of reflected light from the workpiece to determine a filmthickness of the workpiece.

However, the spectrum obtained during polishing of the workpiece islikely to be affected by the polishing liquid present on the polishingpad. For example, under polishing conditions where a flow rate of thepolishing liquid is increased or where a more concentrated polishingliquid is used, the intensity of the reflected light from the workpiecemay decrease, and as a result, the film thickness calculated from thespectrum of the reflected light may differ significantly from an actualfilm thickness.

SUMMARY

Therefore, the present invention provides a polishing method and apolishing apparatus that can reduce influence of polishing liquid (e.g.,slurry) on measuring of a film thickness of a workpiece and can achieveaccurate measuring of the film thickness.

The present invention relates to a technique of polishing a workpiece,such as wafer, substrate, or panel, used in manufacture of semiconductordevices, and in particular to a technique for reducing an effect ofpolishing liquid (e.g., slurry) on measuring of a film thickness of theworkpiece.

In an embodiment, there is provided a polishing method for a workpiece,comprising: supplying polishing liquid onto a polishing pad whilerotating a polishing table supporting the polishing pad; pressing theworkpiece against the polishing pad by a polishing head while thepolishing liquid is present on the polishing pad to polish theworkpiece; during polishing of the workpiece, directing light from anoptical sensor head onto the workpiece and receiving reflected lightfrom the workpiece by the optical sensor head, the optical sensor headbeing disposed in the polishing table; determining a film thickness ofthe workpiece based on a spectrum of the reflected light; and duringpolishing of the workpiece, supplying cleaning liquid from a cleaningnozzle to a target position which is located just above the opticalsensor head, the cleaning nozzle being located above the polishing padand located upstream of the polishing head in a rotating direction ofthe polishing table.

In an embodiment, supplying of the cleaning liquid from the cleaningnozzle is started when the target position is upstream of the polishinghead.

In an embodiment, supplying of the cleaning liquid from the cleaningnozzle is stopped before the target position is moved under thepolishing head.

In an embodiment, the cleaning liquid is intermittently supplied fromthe cleaning nozzle to the target position in synchronization withrotation of the polishing table.

In an embodiment, a supply time of the cleaning liquid per one rotationof the polishing table is less than half a time of one rotation of thepolishing table.

In an embodiment, the polishing method further comprises: supplying gasfrom a gas nozzle to the target position during polishing of theworkpiece, the gas nozzle being located above the polishing pad andlocated upstream of the cleaning nozzle in the rotating direction of thepolishing table.

In an embodiment, the gas and the cleaning liquid are alternatelysupplied to the target position from the gas nozzle and the cleaningnozzle during one rotation of the polishing table.

In an embodiment, there is provided a polishing apparatus for aworkpiece, comprising: a polishing table configured to support apolishing pad; a table motor configured to rotate the polishing table; apolishing-liquid supply nozzle configured to supply polishing liquidonto the polishing pad; a polishing head configured to press theworkpiece against the polishing pad to polish the workpiece while thepolishing liquid is present on the polishing pad; an optical sensor headarranged to direct light to the workpiece and receive reflected lightfrom the workpiece during polishing of the workpiece, the optical sensorhead being disposed in the polishing table; a spectrum processing deviceconfigured to determine a film thickness of the workpiece based on aspectrum of the reflected light; a cleaning nozzle configured to supplycleaning liquid to a target position which is located just above theoptical sensor head, the cleaning nozzle being located above thepolishing pad and located upstream of the polishing head in a rotatingdirection of the polishing table; a cleaning-liquid supply valve coupledto the cleaning nozzle; and a fluid supply controller configured toinstruct the cleaning-liquid supply valve to supply the cleaning liquidfrom the cleaning nozzle to the target position during polishing of theworkpiece.

In an embodiment, the fluid supply controller is configured to instructthe cleaning-liquid supply valve to start supplying the cleaning liquidfrom the cleaning nozzle when the target position is upstream of thepolishing head.

In an embodiment, the fluid supply controller is configured to instructthe cleaning-liquid supply valve to stop supplying the cleaning liquidfrom the cleaning nozzle before the target position moves under thepolishing head.

In an embodiment, the fluid supply controller is configured to instructthe cleaning-liquid supply valve to intermittently supply the cleaningliquid from the cleaning nozzle to the target position insynchronization with rotation of the polishing table.

In an embodiment, a supply time of the cleaning liquid per one rotationof the polishing table is less than half a time of one rotation of thepolishing table.

In an embodiment, the polishing apparatus further comprises a gas nozzleconfigured to supply gas to the target position, the gas nozzle beinglocated above the polishing pad and located upstream of the cleaningnozzle in the rotating direction of the polishing table; and a gassupply valve coupled to the gas nozzle, the fluid supply controllerbeing configured to control operation of the gas supply valve.

In an embodiment, the fluid supply controller is configured to instructthe cleaning-liquid supply valve and the gas supply valve to alternatelysupply the gas and the cleaning liquid from the gas nozzle and thecleaning nozzle to the target position during one rotation of thepolishing table.

In an embodiment, the polishing apparatus further comprises a flow-rateregulating valve configured to regulate a flow rate of the cleaningliquid supplied from the cleaning nozzle to the target position.

According to the embodiments described above, the cleaning liquid issupplied to the target position from above the polishing pad to removethe polishing liquid (e.g., slurry) from the target position. Therefore,the reflected light from the workpiece incident on the optical sensorhead is less affected by the polishing liquid. As a result, an accuratefilm thickness of the workpiece can be determined during polishing ofthe workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an embodiment of a polishingapparatus;

FIG. 2 shows an example of a spectrum;

FIG. 3 shows a top view for illustrating an example of a positionalrelationship between a cleaning nozzle and a polishing head;

FIG. 4 shows a cross-sectional view of a polishing apparatus;

FIG. 5 is a schematic diagram showing another embodiment of thepolishing apparatus; and

FIG. 6 is a schematic diagram showing yet another embodiment of thepolishing apparatus.

DESCRIPTION OF EMBODIMENTS

Embodiments will be described below with reference to the drawings.

FIG. 1 is schematic view showing an embodiment of a polishing apparatus.As shown in FIG. 1 , the polishing apparatus includes a polishing table3 configured to support a polishing pad 2, a polishing head 1 configuredto press a workpiece W (e.g., wafer, substrate, or panel used inmanufacturing of semiconductor devices) against the polishing pad 2, atable motor 6 configured to rotate the polishing table 3, apolishing-liquid supply nozzle 5 configured to supply polishing liquid,such as slurry, onto the polishing pad 2, and a cleaning nozzle 8configured to supply cleaning liquid onto the polishing pad 2. Thepolishing pad 2 has an upper surface constituting a polishing surface 2a for polishing the workpiece W. The workpiece W to be polished in thisembodiment is a circular wafer, while the workpiece W may have othershape, such as rectangular, square, or polygonal shape.

The polishing head 1 is coupled to a head shaft 10, which is coupled toa polishing-head motor (now shown). The polishing-head motor isconfigured to rotate the polishing head 1 together with the head shaft10 in a direction indicated by an arrow. The polishing table 3 iscoupled to the table motor 6, which is configured to rotate thepolishing table 3 and the polishing pad 2 in a direction indicated by anarrow.

Polishing of the workpiece W is performed as follows. Thepolishing-liquid supply nozzle 5 supplies the polishing liquid onto thepolishing surface 2 a of the polishing pad 2 on the polishing table 3,while the polishing table 3 and the polishing head 1 are rotated indirections indicated by the arrows in FIG. 1 . While the workpiece W isbeing rotated by the polishing head 1, the workpiece W is pressed by thepolishing head 1 against the polishing surface 2 a of the polishing pad2 in the presence of the polishing liquid on the polishing pad 2. Thesurface of the workpiece W is polished by a chemical action of thepolishing liquid and a mechanical action of abrasive grains contained inthe polishing liquid and/or the polishing pad 2. The polishing apparatusof this embodiment is a face-down type polishing apparatus configured topress the workpiece W, with its surface to be polished facing downward,against the polishing surface 2 a of the polishing pad 2.

The polishing apparatus includes an optical film-thickness measuringdevice 20 configured to measure a film thickness of the workpiece Wduring polishing of the workpiece W. The optical film-thicknessmeasuring device 20 has a light source 22 configured to emit light, anoptical sensor head 7 configured to direct the light from the lightsource 22 to the workpiece W and receive reflected light from theworkpiece W, a spectrometer 24 configured to measure intensity of thereflected light from the workpiece W, and a spectrum processing device27 configured to determine the film thickness of the workpiece W basedon intensity measurement data of the reflected light from the workpieceW. The light source 22 and the spectrometer 24 are coupled to theoptical sensor head 7. The light source 22, the spectrometer 24, and theoptical sensor head 7 are mounted to the polishing table 3 and rotatetogether with the polishing table 3.

The polishing pad 2 has a through-hole 30 for passing the lighttherethrough. Each time the polishing table 3 makes one rotation, thelight emitted by the light source 22 is transmitted to the opticalsensor head 7 and directed from the optical sensor head 7 through thethrough-hole 30 to the surface of the workpiece W. The light isreflected off the surface of the workpiece W. The reflected light fromthe surface of the workpiece W travels through the through-hole 30 andis received by the optical sensor head 7. The reflected light istransmitted from the optical sensor head 7 to the spectrometer 24. Thespectrometer 24 decomposes the reflected light according to wavelengthover a predetermined wavelength range and measures the intensity of thereflected light at each of the wavelengths to generate intensitymeasurement data of the reflected light. The intensity measurement dataof the reflected light is sent from the spectrometer 24 to the spectrumprocessing device 27.

The spectrum processing device 27 is configured to generate a spectrumof the reflected light of the workpiece W from the intensity measurementdata of the reflected light. This spectrum of the reflected light isexpressed as a line graph (i.e., a spectral waveform) indicating arelationship between the wavelength and the intensity of the reflectedlight. The intensity of the reflected light can also be represented by arelative value, such as a reflectance or a relative reflectance.

The spectrum processing device 27 is configured to determine the filmthickness of the workpiece W based on the spectrum of the reflectedlight. A known technique may be used to determine the film thickness ofthe workpiece W based on the spectrum. For example, the spectrumprocessing device 27 determines, from a reference spectrum library, areference spectrum that is closest in shape to the spectrum of thereflected light and determines a film thickness associated with thedetermined reference spectrum. In another example, the spectrumprocessing device 27 performs a Fourier transform on the spectrum of thereflected light and determines the film thickness from a resultingfrequency spectrum.

FIG. 2 shows an example of a spectrum generated by the spectrumprocessing device 27. The spectrum is represented as a line graph (i.e.,a spectral waveform) showing the relationship between wavelength andintensity of light. In FIG. 2 , horizontal axis represents wavelength ofthe light reflected from the workpiece, and vertical axis representsrelative reflectance derived from the intensity of the reflected light.The relative reflectance is an index value that represents the intensityof the reflected light. Specifically, the relative reflectance is aratio of the intensity of the light to a predetermined referenceintensity. By dividing the intensity of the light (i.e., the actuallymeasured intensity) at each wavelength by a predetermined referenceintensity, unwanted noises, such as a variation in the intensityinherent in an optical system or the light source of the apparatus, areremoved from the actually measured intensity.

In the example shown in FIG. 2 , the spectrum of the reflected light isa spectral waveform showing the relationship between the relativereflectance and the wavelength of the reflected light. The spectrum ofthe reflected light may be a spectral waveform showing a relationshipbetween the intensity itself of the reflected light and the wavelengthof the reflected light.

As shown in FIG. 1 , the spectrum processing device 27 is coupled to apolishing controller 9 for controlling polishing operation for theworkpiece W. The polishing controller 9 is configured to control thepolishing operation for the workpiece W based on the film thickness ofthe workpiece W determined by the spectrum processing device 27. Forexample, the polishing controller 9 is configured to determine apolishing end point at which the film thickness of the workpiece Wreaches a target film thickness, or change polishing conditions for theworkpiece W when the film thickness of the workpiece W reaches apredetermined value.

The spectrum processing device 27 has a memory 27 a storing programstherein and an arithmetic device 27 b configured to perform arithmeticoperations according to instructions contained in the programs. Thespectrum processing device 27 is composed of at least one computer. Thememory 27 a includes a main memory, such as random access memory (RAM),and an auxiliary memory, such as a hard disk drive (HDD) or a solidstate drive (SSD). Examples of the arithmetic device 27 b include a CPU(central processing unit) and a GPU (graphic processing unit). However,the specific configurations of the spectrum processing device 27 are notlimited to these examples.

As shown in FIG. 1 , the cleaning nozzle 8 is disposed above thepolishing table 3 and the polishing pad 2. More specifically, thecleaning nozzle 8 is disposed above a path HP of the optical sensor head7 rotating with the polishing table 3. The cleaning nozzle 8 faces thepath HP. The cleaning nozzle 8 is located upstream of the polishing head1 in a rotating direction of the polishing table 3 and the polishing pad2.

The cleaning nozzle 8 is arranged to supply cleaning liquid from abovethe polishing pad 2 onto the polishing pad 2. More specifically, thecleaning nozzle 8 supplies the cleaning liquid to a target position TPlocated just above the optical sensor head 7. An example of the cleaningliquid is pure water. The target position TP lies in the polishingsurface 2 a of the polishing pad 2. Specifically, the target position TPis a position of the through-hole 30 formed in the polishing pad 2. Inone embodiment, the cleaning liquid may be supplied to an arcuate areaincluding the target position TP just above the optical sensor head 7.The arcuate area is an area along the path HP of the optical sensor head7.

The polishing apparatus has a cleaning-liquid supply line 35 coupled tothe cleaning nozzle 8, a cleaning-liquid supply valve 36 attached to thecleaning-liquid supply line 35, and a fluid supply controller 39configured to control operation of the cleaning-liquid supply valve 36.The cleaning-liquid supply line 35 is coupled to a cleaning-liquidsupply source (not shown) and supplies the cleaning liquid (e.g., purewater) to the cleaning nozzle 8. The cleaning-liquid supply valve 36 isan actuator-driven valve, such as a motor-driven valve, solenoid valve,or air-operated valve. In one embodiment, the cleaning-liquid supplyvalve 36 may be directly coupled to the cleaning nozzle 8.

The fluid supply controller 39 is configured to instruct thecleaning-liquid supply valve 36 to supply the cleaning liquid from thecleaning nozzle 8 onto the target position TP during polishing of theworkpiece W. The cleaning liquid released from the cleaning nozzle 8 canremove the polishing liquid from the target position TP. Therefore, thereflected light from the workpiece W incident on the optical sensor head7 is less affected by the polishing liquid, and as a result, thespectrum processing device 27 can accurately determine the filmthickness of the workpiece W during polishing of the workpiece W.

The polishing apparatus further includes a flow-rate regulating valve 37attached to the cleaning-liquid supply line 35. The flow-rate regulatingvalve 37 is located upstream of the cleaning-liquid supply valve 36 andis configured to regulate a flow rate of the cleaning liquid to besupplied from the cleaning nozzle 8. The flow-rate regulating valve 37is coupled to the fluid supply controller 39, and operation of theflow-rate regulating valve 37 is controlled by the fluid supplycontroller 39. The operation of the flow-rate regulating valve 37, i.e.,the flow rate of the cleaning liquid to be supplied from the cleaningnozzle 8 to the target position TP, is set based on rotational speed ofthe polishing table 3, type of the polishing pad 2, type and/orconcentration of the cleaning liquid, and structure of the polishing pad2 (e.g., with or without a transparent window, as described below).

During polishing of the workpiece W, both the polishing liquid and thecleaning liquid are supplied onto the polishing pad 2. Specifically,while the polishing liquid (e.g., slurry) is supplied onto the polishingsurface 2 a of the polishing pad 2, the cleaning liquid is supplied tothe target position TP in the polishing pad 2. In order to prevent thecleaning liquid once supplied to the target position TP from beingreplaced with the polishing liquid, it is preferable to supply thecleaning liquid from the cleaning nozzle 8 to the target position TPwhen the target position TP approaches the polishing head 1.

Thus, the fluid supply controller 39 is configured to instruct thecleaning-liquid supply valve 36 to start supplying of the cleaningliquid from the cleaning nozzle 8 when the target position TP isupstream of the polishing head 1 during polishing of the workpiece W. Inone embodiment, the fluid supply controller 39 is configured to instructthe cleaning-liquid supply valve 36 during polishing of the workpiece Wto stop supplying of the cleaning liquid from the cleaning nozzle 8before the target position TP moves under the polishing head 1. Suchcleaning liquid supply operations make it possible to prevent thepolishing liquid from being diluted with the cleaning liquid and tothereby prevent a decrease in polishing rate (which may be referred toas removal rate) of the workpiece W.

In this embodiment, the fluid supply controller 39 is configured toinstruct the cleaning-liquid supply valve 36 to intermittently supplythe cleaning liquid from the cleaning nozzle 8 to the target position TPin synchronization with the rotation of the polishing table 3. Morespecifically, each time the polishing table 3 makes one rotation, thesupply of the cleaning liquid is started and stopped. Each time thepolishing table 3 makes one rotation, the supply of the gas may bestarted and stopped multiple times to remove the polishing liquid fromthe target position TP by the pressure of the cleaning liquid. A supplytime of the cleaning liquid per one rotation of the polishing table 3 isshorter than half a time of one rotation of the polishing table 3. Inone embodiment, from a viewpoint of preventing dilution of the polishingliquid, the supply time of the cleaning liquid per one rotation of thepolishing table 3 is shorter than one-third of the time of one rotationof the polishing table 3.

The fluid supply controller 39 has a memory 39 a storing programstherein and an arithmetic device 39 b configured to perform arithmeticoperations according to instructions contained in the programs. Thefluid supply controller 39 is composed of at least one computer. Thememory 39 a includes a main memory, such as random access memory (RAM),and an auxiliary memory, such as a hard disk drive (HDD) or a solidstate drive (SSD). Examples of the arithmetic device 39 b include a CPU(central processing unit) and a GPU (graphic processing unit). However,the specific configurations of the fluid supply controller 39 are notlimited to these examples.

FIG. 3 shows a top view for illustrating an example of a positionalrelationship between the cleaning nozzle 8 and the polishing head 1.Where a straight line extending from a center of rotation CP of thepolishing table 3 to an upstream end surface of the polishing head 1 isdenoted by L1, and a straight line extending from the center of rotationCP of the polishing table 3 to an outlet of the cleaning nozzle 8 isdenoted by L2, an angle α between the straight line L1 and the straightline L2 is in a range of 0 to 120 degrees, preferably 20 to 90 degrees,more preferably 30 to 60 degrees. The cleaning nozzle 8 in such aposition can emit the cleaning liquid onto the target position TP justbefore the target position TP moves under the polishing head 1. As aresult, the cleaning liquid once supplied to the target position TP isprevented from being replaced with the polishing liquid.

Details of the optical film-thickness measuring device 20 are nowdescribed with reference to FIG. 4 . The spectrometer 24 has a lightdetector 25. In one embodiment, the light detector 25 comprises aphotodiode, CCD, or CMOS. The optical sensor head 7 is optically coupledto the light source 22 and the light detector 25. The light detector 25is electrically coupled to the spectrum processing device 27.

The optical film-thickness measuring device 20 includes a light-emittingoptical fiber cable 41 arranged to direct the light, emitted by thelight source 22, to the surface of the workpiece W, and alight-receiving optical fiber cable 42 arranged to receive the reflectedlight from the workpiece W and transmit the reflected light to thespectrometer 24. A distal end of the light-emitting optical fiber cable41 and a distal end of the light-receiving optical fiber cable 42 arelocated in the polishing table 3.

The distal end of the light-emitting optical fiber cable 41 and thedistal end of the light-receiving optical fiber cable 42 constitute theoptical sensor head 7 that directs the light to the surface of theworkpiece W and receives the reflected light from the workpiece W. Theother end of the light-emitting optical fiber cable 41 is coupled to thelight source 22, and the other end of the light-receiving optical fibercable 42 is coupled to the spectrometer 24. The spectrometer 24 isconfigured to decompose the reflected light from the workpiece Waccording to wavelength and measure intensities of the reflected lightover a predetermined wavelength range.

The polishing table 3 has a first hole 50A and a second hole 50B whichare open in an upper surface of the polishing table 3. The polishing pad2 has the through-hole 30 formed at a position corresponding to theseholes 50A, 50B. The holes 50A, 50B communicate with the through-hole 30,and the through-hole 30 is open in the polishing surface 2 a. The firsthole 50A is coupled to a liquid supply line 54, and the second hole 50Bis coupled to a drain line 55. The optical sensor head 7, which isconstituted of the distal end of the light-emitting optical fiber cable41 and the distal end of the light-receiving optical fiber cable 42, islocated in the first hole 50A and below the through-hole 30.

During polishing of the workpiece W, pure water is supplied as a rinsingliquid to the first hole 50A through the liquid supply line 54, and isfurther supplied through the first hole 50A to the through-hole 30. Thepure water fills a space between the surface of the workpiece W (thesurface to be polished) and the optical sensor head 7. The pure waterflows into the second hole 50B and is discharged through the drain line55. The pure water flowing through the first hole 50A and thethrough-hole 30 prevents the polishing liquid from entering the firsthole 50A, thereby ensuring an optical path.

The light emitted from the optical sensor head 7 travels through thethrough-hole 30 onto the workpiece W, and the reflected light from theworkpiece W passes through the through-hole 30 and is received by theoptical sensor head 7. In this embodiment, the position of thethrough-hole 30 is the target position TP where the cleaning liquid isto be supplied from the cleaning nozzle 8. Therefore, the cleaningnozzle 8 supplies the cleaning liquid to the through-hole 30, and thecleaning liquid can remove the polishing liquid (e.g., slurry) from thethrough-hole 30. In particular, according to this embodiment, both thepure water supplied from the liquid supply line 54 and the cleaningliquid supplied from the cleaning nozzle 8 can remove the polishingliquid existing over the optical sensor head 7.

The light-emitting optical fiber cable 41 is a light-transmittingelement that directs the light, emitted by the light source 22, to thesurface of the workpiece W. The distal ends of the light-emittingoptical fiber cable 41 and the light-receiving optical fiber cable 42are located in the first hole 50A and located near the surface, to bepolished, of the workpiece W. The optical sensor head 7, which isconstituted of the distal ends of the light-emitting optical fiber cable41 and the light-receiving optical fiber cable 42, is arranged so as toface the workpiece W held on the polishing head 1. Each time thepolishing table 3 makes one rotation, the light is emitted ontopredetermined measurement point(s) on the workpiece W. In thisembodiment, only one optical sensor head 7 is provided in the polishingtable 3, while a plurality of optical sensor heads 7 may be provided inthe polishing table 3. For example, two optical sensor heads 7 may beprovided so as to sweep across a center and an edge of the workpiece W,respectively.

FIG. 5 is a schematic diagram of another embodiment of the polishingapparatus. Configurations and operations of this embodiment, which willnot be described particularly, are the same as those of the embodimentsdescribed with reference to FIGS. 1 through 4 , and redundantdescriptions thereof are omitted. In the embodiment shown in FIG. 5 ,the polishing apparatus further includes a gas nozzle 61 configured tosupply gas to the target position TP, a gas supply line 62 coupled tothe gas nozzle 61, and a gas supply valve 63 attached to the gas supplyline 62. Examples of the gas include air and inert gas (e.g., nitrogengas).

The gas supply line 62 is coupled to a gas supply source (not shown) andsupplies the gas to the gas nozzle 61. The gas supply valve 63 is anactuator-driven valve, such as a motor-driven valve, solenoid valve, orair-operated valve. In one embodiment, the gas supply valve 63 may bedirectly coupled to the gas nozzle 61.

The gas nozzle 61 is located above the polishing table 3 and thepolishing pad 2. More specifically, the gas nozzle 61 is located abovethe path HP of the optical sensor head 7 that rotates together with thepolishing table 3. The gas nozzle 61 faces the path HP. The gas nozzle61 is located upstream of the cleaning nozzle 8 in the rotatingdirection of the polishing table 3 and the polishing pad 2. The gasnozzle 61 is arranged to supply the gas from above the polishing pad 2onto the polishing pad 2. More specifically, the gas nozzle 61 suppliesthe gas to the target position TP which is located just above theoptical sensor head 7.

The fluid supply controller 39 is configured to control the operation ofthe gas supply valve 63 in addition to the cleaning-liquid supply valve36. The fluid supply controller 39 is configured to instruct the gassupply valve 63 to supply the gas from the gas nozzle 61 to the targetposition TP during polishing of the workpiece W. More specifically, thefluid supply controller 39 is configured to instruct the cleaning-liquidsupply valve 36 and the gas supply valve 63 to alternately supply thegas and the cleaning liquid from the gas nozzle 61 and the cleaningnozzle 8 to the target position TP during one rotation of the polishingtable 3.

During one rotation of the polishing table 3, the gas is first suppliedto the target position TP, and the cleaning liquid is then supplied tothe target position TP. Jet of the gas can remove the polishing liquidpresent at the target position TP. The cleaning liquid can not onlyremove the residual polishing liquid at the target position TP, but canalso return a dried portion of the polishing pad 2 as a result of beingexposed to the jet of the gas to a wet state.

The gas, as well as the cleaning liquid, can remove the polishing liquidfrom the target position TP, but on the other hand, the supply of thegas may dry the polishing pad 2. The dried polishing pad 2 may causescratches on the workpiece W. According to this embodiment, after thegas is supplied to the polishing pad 2, the polishing pad 2 is returnedto the wet state by the cleaning liquid, so that a dried state of thepolishing pad 2 can be prevented.

In one embodiment, the fluid supply controller 39 is configured toinstruct the gas supply valve 63 to stop the emission of the gas fromthe gas nozzle 61 during polishing of the workpiece W before the targetposition TP moves under the polishing head 1. Such gas supply operationcan reduce the amount of gas supplied for removing the polishing liquid.

In this embodiment, the fluid supply controller 39 is configured toinstruct the gas supply valve 63 and the cleaning-liquid supply valve 36to alternately and intermittently supply the gas and the cleaning liquidfrom the gas nozzle 61 and the cleaning nozzle 8 to the target positionTP in synchronization with the rotation of the polishing table 3. Morespecifically, each time the polishing table 3 makes one rotation, thesupply of the gas is started and stopped, and the supply of the cleaningliquid is started and stopped. A supply time of the gas per one rotationof the polishing table 3 is shorter than half a time of one rotation ofthe polishing table 3. From the viewpoint of preventing the polishingpad 2 from being dried, the supply time of the gas per one rotation ofthe polishing table 3 is shorter than one-third of the time of onerotation of the polishing table 3. Each time the polishing table 3 makesone rotation, the supply of the gas may be started and stopped multipletimes to remove the polishing liquid from the target position TP by thepressure of the jet of the gas.

FIG. 6 is a schematic diagram showing another embodiment of thepolishing apparatus. Configurations and operations of this embodiment,which will not be described particularly, are the same as those of theembodiments described with reference to FIGS. 1 through 4 , andredundant descriptions thereof are omitted. In the embodiment shown inFIG. 6 , the polishing apparatus has a transparent window 70 disposed inthe polishing pad 2, instead of the liquid supply line 54 and the drainline 55. The transparent window 70 is disposed in the through-hole 30formed in the polishing pad 2. The transparent window 70 completelycloses the through-hole 30 of the polishing pad 2, so that thetransparent window 70 can prevent the polishing liquid and polishingdebris from coming into contact with the optical sensor head 7.

The transparent window 70 is located just above the optical sensor head7. Therefore, the optical sensor head 7 emits the light through thetransparent window 70 onto the workpiece W and receives the reflectedlight from the workpiece W that has passed through the transparentwindow 70. The transparent window 70 is composed of a material that cantransmit the light therethrough. The material of the transparent window70 is not limited, but for example, it is composed of a transparentresin.

In this embodiment, the target position TP to which the cleaning liquidshould be supplied is a position of an upper surface of the transparentwindow 70. As well as the embodiments described with reference to FIGS.1 to 4 , the cleaning nozzle 8 supplies the cleaning liquid onto thetarget position TP, i.e., onto the upper surface of the transparentwindow 70, during polishing of the workpiece W. The cleaning liquidemitted from the cleaning nozzle 8 can remove the polishing liquid fromthe target position TP, i.e., the upper surface of the transparentwindow 70. Therefore, the reflected light from the workpiece W incidenton the optical sensor head 7 is less affected by the polishing liquid,and as a result, the spectrum processing device 27 can accuratelydetermine the film thickness of the workpiece W during polishing of theworkpiece W.

The embodiment of the transparent window 70 shown in FIG. 6 isapplicable to the embodiments having both the gas nozzle 61 and thecleaning nozzle 8 described with reference to FIG. 5 . The arrangementof the gas nozzle 61 and cleaning nozzle 8 and the supply timing of gasand liquid are the same as in the embodiments described with referenceto FIG. 5 , and their redundant descriptions are omitted.

The previous description of embodiments is provided to enable a personskilled in the art to make and use the present invention. Moreover,various modifications to these embodiments will be readily apparent tothose skilled in the art, and the generic principles and specificexamples defined herein may be applied to other embodiments. Therefore,the present invention is not intended to be limited to the embodimentsdescribed herein but is to be accorded the widest scope as defined bylimitation of the claims.

What is claimed is:
 1. A polishing method for a workpiece, comprising: supplying polishing liquid onto a polishing pad while rotating a polishing table supporting the polishing pad; pressing the workpiece against the polishing pad by a polishing head while the polishing liquid is present on the polishing pad to polish the workpiece; during polishing of the workpiece, directing light from an optical sensor head onto the workpiece and receiving reflected light from the workpiece by the optical sensor head, the optical sensor head being disposed in the polishing table; determining a film thickness of the workpiece based on a spectrum of the reflected light; and during polishing of the workpiece, supplying cleaning liquid from a cleaning nozzle to a target position which is located just above the optical sensor head, the cleaning nozzle being located above the polishing pad and located upstream of the polishing head in a rotating direction of the polishing table.
 2. The polishing method according to claim 1, wherein supplying of the cleaning liquid from the cleaning nozzle is started when the target position is upstream of the polishing head.
 3. The polishing method according to claim 2, wherein supplying of the cleaning liquid from the cleaning nozzle is stopped before the target position is moved under the polishing head.
 4. The polishing method according to claim 1, wherein the cleaning liquid is intermittently supplied from the cleaning nozzle to the target position in synchronization with rotation of the polishing table.
 5. The polishing method according to claim 4, wherein a supply time of the cleaning liquid per one rotation of the polishing table is less than half a time of one rotation of the polishing table.
 6. The polishing method according to claim 1, further comprising: supplying gas from a gas nozzle to the target position during polishing of the workpiece, the gas nozzle being located above the polishing pad and located upstream of the cleaning nozzle in the rotating direction of the polishing table.
 7. The polishing method according to claim 6, wherein the gas and the cleaning liquid are alternately supplied to the target position from the gas nozzle and the cleaning nozzle during one rotation of the polishing table.
 8. A polishing apparatus for a workpiece, comprising: a polishing table configured to support a polishing pad; a table motor configured to rotate the polishing table; a polishing-liquid supply nozzle configured to supply polishing liquid onto the polishing pad; a polishing head configured to press the workpiece against the polishing pad to polish the workpiece while the polishing liquid is present on the polishing pad; an optical sensor head arranged to direct light to the workpiece and receive reflected light from the workpiece during polishing of the workpiece, the optical sensor head being disposed in the polishing table; a spectrum processing device configured to determine a film thickness of the workpiece based on a spectrum of the reflected light; a cleaning nozzle configured to supply cleaning liquid to a target position which is located just above the optical sensor head, the cleaning nozzle being located above the polishing pad and located upstream of the polishing head in a rotating direction of the polishing table; a cleaning-liquid supply valve coupled to the cleaning nozzle; and a fluid supply controller configured to instruct the cleaning-liquid supply valve to supply the cleaning liquid from the cleaning nozzle to the target position during polishing of the workpiece.
 9. The polishing apparatus according to claim 8, wherein the fluid supply controller is configured to instruct the cleaning-liquid supply valve to start supplying the cleaning liquid from the cleaning nozzle when the target position is upstream of the polishing head.
 10. The polishing apparatus according to claim 9, wherein the fluid supply controller is configured to instruct the cleaning-liquid supply valve to stop supplying the cleaning liquid from the cleaning nozzle before the target position moves under the polishing head.
 11. The polishing apparatus according to claim 8, wherein the fluid supply controller is configured to instruct the cleaning-liquid supply valve to intermittently supply the cleaning liquid from the cleaning nozzle to the target position in synchronization with rotation of the polishing table.
 12. The polishing apparatus according to claim 11, wherein a supply time of the cleaning liquid per one rotation of the polishing table is less than half a time of one rotation of the polishing table.
 13. The polishing apparatus according to claim 8, further comprising: a gas nozzle configured to supply gas to the target position, the gas nozzle being located above the polishing pad and located upstream of the cleaning nozzle in the rotating direction of the polishing table; and a gas supply valve coupled to the gas nozzle, the fluid supply controller being configured to control operation of the gas supply valve.
 14. The polishing apparatus according to claim 13, wherein the fluid supply controller is configured to instruct the cleaning-liquid supply valve and the gas supply valve to alternately supply the gas and the cleaning liquid from the gas nozzle and the cleaning nozzle to the target position during one rotation of the polishing table.
 15. The polishing apparatus according to claim 8, further comprising a flow-rate regulating valve configured to regulate a flow rate of the cleaning liquid supplied from the cleaning nozzle to the target position. 